Do at least Act 1 of the Main Quest early as there's shout rewards. Then you can go off wandering for as long as you want. Tip straight from the guide.
And if it's not known already, resting actually does something in this game unlike Oblivion. You get 8 hrs odd of performance boost, don't know yet if more or less rest = more or less boost so experiment. But get some kip when you can.
Here's what the guide says - "blah blah blah.. it is recommended that you complete most of the first act sooner rather than later, as you're rewarded with several important powers (inc Shouts)... blah, blah, blah"
Also notes "you are free to continue or ignore the Main Quest whenever you wish"
For the budding Archery enthusiast who is okay with abusing exploits:
You can recruit a companion named Faendol very early in the game. He is also an archery trainer. You can pay him for archery training, then talk to him and choose the dialogue option that allows you to trade items with him, and take your money right back
from him. I'm unsure how high he can train you because I don't abuse exploits, but there it is.